The purpose of this investigation is to further elucidate the structure and control of biosynthesis of IgA, the major class of immunoglobulins of the secretory fluids. We have established that there are two subclasses of IgA in the rabbit. All four allotypes of IgA-f subclass are resistant to proteolytic digestion, all five allotypes of the IgA-g subclass are susceptible to cleavage by various proteolytic enzymes. Only the IgA-g sublcass of secretory Iga (sIgA) has non- covalently bound secretory component (SC). We are continuing studies to determine the structural basis for subclass differences and differences in SC-IgA interactions. Numerous cross-reactions have been noted among the IgA-g allotypes and among the IgA-f allotypes with various anti- allotype antisera. Moreover, cross-reactions have been found between human Ig and rabbit Ig with rabbit anti-allotype antisera. Chemical and immunochemical studies are in progress to elucidate the nature of these cross-reacting antigenic determinants. Quantitative studies are being done to determine the factors regulating the expression of genes for the IgA allotypes and subclasses in rabbits with various combinations of genetically defined Ig heavy chain chromosomal regions. In vivo and in vitro experiments are proposed to study the development and regulation of the IgA system at the cellular level.